A RARE TYPE OF VIADUCT, BUILT 163 years ago, will eventually carry trains traveling at speeds reaching 140 miles per hour as a contractor plugs ahead on a complex rehabilitation in Massachusetts.

The Canton Viaduct, a multiple-arch masonry structure, originally carried a single track, but a second track was added over the years for trains run by Amtrak and the Massachusetts Bay Transportation Authority, says David Peters, principal engineer for Omaha-based designer HDR Engineering Inc. Steel railing was also added to partially retain the ballast that holds the tracks in place. ``The MBTA has had to operate commuter rail with a speed restriction of 20 mph'' over the viaduct, he says.

Chelmsford, Mass-based Middlesex Corp. holds the approximately $9-million contract to widen, lengthen and strengthen the 583-ft-long viaduct so that high-speed Northeast Corridor trains can one day cross it at 140 mph while retaining the historic look. The viaduct, which has 21 spans, will be lengthened to 643 ft with the addition of a span at each end and be widened by about 4 ft, says Paul Delinda, Middlesex operations manager. A chain-link fence is all that separates workers from 53 trains a day.

Construction began last year after intense design and investigation of the 22-ft-wide structure, which spans the Neponset River and reaches 50 ft in height. Secondary concrete arches installed in 1910 below existing arches ``had deteriorated substantially,'' and had to be replaced, says Peters.

About 200 minipiles are being drilled through existing buttresses at each pier into bedrock, as much as 80 ft deep. Then, precast, prestressed tub-shaped beams, each about 48 tons and 28 ft long, are installed between the piers, supported on the minipiles. The tight tolerances and 1 curve of the viaduct make installation difficult. ``One track has to be maintained at all times,'' says Vahid Ownjazayeri, MBTA deputy director of construction. ``They're driving minipiles, pouring concrete and removing stone all with close proximity to the busiest corridor in Massachusetts.'' A 300-ton crane at times must lift the beams over the opposite side of the track into place. Middlesex also built a 45-ft-high, 600-ft-wide scaffold for the job, says Delida.

The original steel railing is being sandblasted and galvanized, while old concrete is chipped out and replaced. The reinstalled railing will have a new safety walkway stretching the length of the viaduct, and inserts for future installation of poles carrying electrical wires for the high-speed trains, says Peters.

Much of the work is done between 2 and 4 a.m. to avoid commuter traffic. The contractor is just about done installing the 22 beams for one side of the viaduct, and this spring will begin demolition and reconstruction of the other side, says David Skerrett, Middlesex vice president of construction. The job should be completed by the end of the year.

Copyright © 1998 The McGraw-Hill Companies, Inc.